Our work concerns the use of a live virus vaccine for influenza A as an antiviral therapeutic agent. This live virus vaccine has been developed and shown to be a safe and effective vaccine by others. We have found that this live virus vaccine (in addition to its ability to induce a protective immune response) is a dominant mutant that directly interferes with the growth of epidemic strains of influenza when tested in mixed infection in cell culture. Moreover, the vaccine virus is able to protect animals from disease caused by epidemic strains of influenza under conditions of simultaneous infection with equal quantities of the two viruses. Preliminary results in human trails suggest that the vaccine virus will act as an antiviral agent in humans as it does in animals. We have identified the gene of the vaccine virus responsible for the interference trait. We would like to identify the nucleic acid sequences responsible for this trait by comparison of the vaccine virus se quence with a variety of other closely related influenza A viruses whose sequences are known.